Memory: Remove count of cached pages and add InvalidateRegion

In a future commit, the count of cached pages will be reintroduced in
the actual surface cache. Also adds an Invalidate only to the cache
which marks a region as invalid in order to try to avoid a costly flush
from 3ds memory

src/core/hle/kernel/vm_manager.cpp

@@ -58,7 +58,6 @@ void VMManager::Reset() {
 
     page_table.pointers.fill(nullptr);
     page_table.attributes.fill(Memory::PageType::Unmapped);
-    page_table.cached_res_count.fill(0);
 
     UpdatePageTableForVMA(initial_vma);
 }